1. Industrial Application Field
This invention pertains to a method for manufacturing ferroelectric capacitors (especially ferroelectric capacitors with a lead zirconate titanate (PZT) film) and a method for manufacturing ferroelectric memory devices (especially nonvolatile semiconductor memories using the ferroelectric capacitor with a PZT film).
2. Prior Art
By forming a capacitor using a film made of PZT, a ferroelectric material, as the dielectric film, it is possible to manufacture a nonvolatile memory element having a simple configuration and using the remnant polarization characteristics of the capacitor, that is, a ferroelectric RAM (ferroelectric random access memory) known as a nonvolatile memory called FRAM.
In the conventional FRAM, however, the so-called xe2x80x9cpolarization fatiguexe2x80x9d phenomenon, which will significantly deteriorate the polarization characteristics, tends to occur because the inversion (that is, the polarization inversion performed when data are written or read) is repeated during the operation of the PZT capacitor Cap. This phenomenon causes problems in developing practical devices. When the inversion is repeated about 1,000,000 times, the polarization intensity is reduced to about half of the original value or even less. Consequently, it has been difficult to develop devices with high reliability for repeated reading or writing operation.
Progress for Achieving this Invention
In Japanese Patent Application No. Hei 8[1996]-181358, the present inventors have already proposed a novel effective method (referred to as the previous invention hereinafter) that can alleviate the aforementioned polarization fatigue under two conditions. According to the previous invention, the first condition is to use and crystallize a PZT feed solution (used when forming a film by the sol-gel method) containing excess lead at a temperature higher than 650xc2x0 C. The second condition is to use electrodes made of Ir, Ru, or other oxidizing metals.
The aforementioned first condition will be explained as follows. When amorphous PZT is deposited on a lower electrode made of Ir and is heated to a temperature higher than the crystallization temperature, crystallization is carried out in the thickness direction. A columnar grain structure is formed during the crystallization process, and the excess lead is pressed out to the surface to form a structural transition layer mainly composed of the Pb on the surface of the PZT layer.
In this case, the composition of a preferred PZT precursor solution is such that Pb=1.02-1.50 (with respect to Zr+Ti=1.0) (the ratio of Ti/Zr can be selected at will). If the Pb concentration is too low, it is difficult to form the aforementioned columnar structure (control of the PZT crystallization direction). On the other hand, if the Pb concentration is too high, the aforementioned structural transition layer becomes too thick to be removed by heat treatment.
When the sintering temperature in the heat treatment is too low (600xc2x0 C.), it is easy to form the aforementioned structural transition layer mainly composed of Pb. On the other hand, the structural transition layer will disappear if the sintering temperature is controlled to 650xc2x0 C. or higher. However, if the sintering temperature is too high, it is difficult to form the PZT crystal. Consequently, the sintering temperature should be maintained at 750xc2x0 C. or lower.
As far as the sintering (annealing) temperature for PZT is concerned, for the PZT formed at 600xc2x0 C., the remnant polarization intensity is almost reduced to 0 after 100,000,000 polarization inversions. On the other hand, the polarization characteristics of the capacitor formed at 650xc2x0 C. are improved significantly. Also, the remnant polarization intensity of a sample formed at a temperature in the range of 650-700xc2x0 C. is barely reduced after 100,000,000 inversions. The reason for the improvement in the fatigue characteristic property is that the structural transition layer made of the excess pB on the surface disappears at a temperature of 650xc2x0 C. or higher.
In the following, the electrode material as the aforementioned second condition will be explained. Generally, Pt or another substance that cannot be oxidized is used for the electrodes of the PZT capacitor (in some cases, however, Au is used for the upper electrode alone). The previous invention has succeeded in alleviating the polarization fatigue by using metal Ir for both the lower and upper electrodes. For example, the polarization characteristics of acapacitor using Pt for its electrodes deteriorate significantly when the inversion is repeated more than 2xc3x97108 times. On the other hand, when Ir is used for the electrodes, deterioration in the polarization characteristics does not occur until the inversion is repeated 2xc3x97109 times.
As described above, the polarization fatigue characteristic property has a strong dependency on the material of the electrodes. Compared with other types of capacitors, the capacitor with both upper and lower electrodes made of Ir has a very stable remnant polarization intensity (Pr) during the polarization inversion. It is believed that the reason for the stable remnant polarization intensity is due to the oxidizability of the Ir metal.
Problems to be Solved by the Invention
While the previous invention has the aforementioned desirable features, there is still a problem remaining to be solved. That is, in a manufacturing process that satisfies the aforementioned conditions, it is difficult to keep the temperature low because the temperature (sintering temperature) for forming the PZT has to be kept at the level of 650xc2x0 C. or higher.
When the temperature is high, the element region and wiring on the semiconductor substrate will be affected by the generated heat. For example, a change in the concentration of the impurities or damage of Al wiring might occur.
The purpose of this invention is to provide a method that can take advantage of the features of the previous invention and can also be used to manufacture capacitors free of polarization fatigue, even when the treatment is carried out at a low temperature.
Means to Solve the Problems
This invention provides a method for manufacturing a ferroelectric capacitor characterized by the fact that the process for forming the ferroelectric capacitor by sequentially laminating a ferroelectric film and a first electrode on a second electrode (for example, the process for forming the ferroelectric capacitor by sequentially laminating a lead zirconate titanate layer and an upper electrode made of iridium, iridium oxide, rubidium, rubidium oxide, platinum, or palladium on a lower electrode made of iridium, iridium oxide, rubidium, rubidium oxide, platinum, or palladium) has the following steps:
a step in which a ferroelectric material layer containing an excess amount of the specific constituent element of the aforementioned ferroelectric film is formed on the aforementioned first electrode (for example, a step in which an amorphous layer of lead zirconate titanate containing excess lead is formed on the aforementioned lower electrode);
a step in which the aforementioned ferroelectric material is crystallized by a heat treatment to form the aforementioned ferroelectric film (for example, a step in which the aforementioned amorphous layer is crystallized by a heat treatment to form a crystal layer of lead zirconate titanate);
a step in which the surface deposit mainly composed of the aforementioned specific constituent element and generated on the surface of the aforementioned ferroelectric film during the crystallization process is removed (for example, a step in which the surface deposit mainly made of the excess lead and generated on the surface of the aforementioned crystal layer during the crystallization process is removed by means of etching or dissolution); and
a step in which the aforementioned upper electrode is formed on the aforementioned crystal layer exposed after the aforementioned surface deposit is removed.
This invention also provides a method for manufacturing a ferroelectric memory device characterized by the fact that the ferroelectric capacitor in a memory cell is formed using the aforementioned method.
According to this invention, a ferroelectric material layer, such as an amorphous layer made of lead zirconate titanate containing an excessive amount of the specific constituent element of the aforementioned ferroelectric film, such as lead, is formed on the aforementioned first electrode. Then, the ferroelectric material is crystallized by a heat treatment to form the aforementioned ferroelectric film. After that, the surface deposit, such as the aforementioned structural transition layer made of the excess pB, mainly composed of the aforementioned specific constituent element and generated on the surface of the aforementioned ferroelectric film during the crystallization process, is removed. Consequently, even when the aforementioned heat treatment is performed at a low temperature (especially 600xc2x0 C. or below), the deterioration in the polarization fatigue characteristic caused by the surface deposit can be prevented because the generated surface deposit is removed by the aforementioned heat treatment. In-addition, by using the material layer containing an excessive amount of the specific constituent element, such as Pb, the crystallization can be carried out in the desired direction to form a ferroelectric film with a good polarization inversion characteristic.
Since it becomes possible to perform the heat treatment at a low temperature, the influence of the heat on the element region and wiring of the semiconductor substrate can be minimized. Consequently, the change in the concentration of the impurities and the damage of Al wiring can be prevented.
In the method of this invention, the aforementioned surface deposit can be removed by means of dry etching or chemical dissolution performed after the surface deposit is vitrified. When the aforementioned surface deposit is removed by means of dry etching, it is preferred that a heat treatment be performed at 400-700xc2x0 C. after the dry etching operation to recover from damage to the ferroelectric film caused by dry etching.
When the aforementioned amorphous layer containing excess pB is used as the material of the ferroelectric film, it is preferred that the content of lead in the aforementioned amorphous layer made of lead zirconate titanate be 1.02-1.50 times (in the ratio of number of atoms) as much as the total amount of zirconium and titanium. It is preferred that the aforementioned heat treatment be performed at a low temperature of 600xc2x0 C. or below.
In the method of this invention, in order to simplify the manufacturing process, it is preferred to form an insulating layer on the surface of the substrate before the aforementioned surface deposit is removed. After the prescribed portions of the aforementioned insulating layer are removed, the aforementioned surface deposit right below the removed portions is removed to expose part of the aforementioned ferroelectric film.
In this case, the prescribed portions of the aforementioned insulating layer on the surface of the substrate and the aforementioned surface deposit below the prescribed portions can be removed by means of etching or dissolution using a common mask. The aforementioned upper electrode is formed on the exposed crystal layer. The aforementioned insulating layer and surface deposit can be removed by means of dry etching or chemical dissolution performed after the surface deposit is vitrified.
After the dry etching operation, it is preferred that a heat treatment be performed at a temperature in the range of 400-700xc2x0 C. to recover from damage caused by the dry etching.
It is preferred that the content of lead in the aforementioned amorphous layer made of lead zirconate titanate be 1.02-1.50 times (in the ratio of number of atoms) as much as the total amount of zirconium and titanium. In addition, it is preferred that the aforementioned heat treatment be performed at a low temperature of 600xc2x0 C.